1. Field of The Invention
The invention relates to a controlled clearance piston load cell and to an apparatus for calibrating force-measuring devices which utilizes a controlled clearance piston load cell as a primary standard.
2. Description of Related Art
U.S. Pat. No. 3,296,855, issued Jan. 10, 1967 to D. H. Newhall describes a controlled clearance piston load cell which is capable of extremely accurate and dependable measurements over a range from few to several million pounds. The load cell includes a base, a metallic cylinder supported on the base to rotate on a vertical axis, a piston disposed within and having a leakage fit clearance with the cylinder for movement thereto, and a plug disposed within the cylinder below the piston having a leakage fit clearance with the cylinder and forming with the base a bottom closure for the cylinder. Pressurized fluid is introduced to the cylinder between the piston and the plug to balance the piston against an external force to be measured, which acts on the top end of the piston. The cylinder is surrounded by a jacket which forms an annular cavity extending almost the entire length of the cylinder. Pressurized fluid is applied to this cavity to contract the cylinder radially and maintain the proper leakage fit clearance between the cylinder and the piston and between the cylinder and the plug. This load cell does not allow any adjustment for load misalignment. Consequently, a rotary oscillatory or vibratory movement must be imparted to the cylinder while the load cell is in use to free the piston of any tendency to stick to the interior wall of the cylinder and thus insure a free floating relationship between the piston and cylinder. Also, great care must be taken to assure proper alignment of the loading apparatus used with this load cell.
In the past, force gages have been calibrated by one of three methods: (1) the application of calibrated weights to the load surface of the gage; (2) the interposition of calibrated load rings between the gage and the load; and (3) the direct comparison of the gage response to the response of another gage or device under the same load.
The accepted primary standard for calibrating a force gage is the dead-weight method, which subjects the gage to a precisely known amount of dead weight. The National Bureau of Standards (NBS) uses this technique for forces ranging to 1,000,000 pounds. Dead-weight calibration systems are loading frames whose upper cross-members form a pendulum in conjunction with the load weights attached at the bottom. These machines require shelters which are several stories high and must be climatically controlled. Consequently, this technique requires a massive initial investment as well as large maintenance and operational costs. This technique for large forces ia available as a primary standard soley at NBS at great expense to the user. This technique does not economically lend itself to implementation in small calibration or production facilities.
A more commonly used method to calibrate testing equipment and load cells is the load ring method. The load ring is a short, hollow, steel cylinder with two diametrically located saddle bosses used to apply an unknown load perpendicular to the cylinder's longitudinal axis. As the load is increased, the length of the internal diameter which passes through the center of the bosses is measured by various means until a predetermined shortening is achieved. By previous calculation and calibration, this change in diameter represents a precise load. While this method is fine for static calibrations of particular points in both tension and compression, the inherent nonlinearity of the rings structural geometry render them unsuitable for dynamic calibration. In order to use ring loads over a wide range of applied forces, numerous sets of rings are required. Because of their thermal stress response and sensitivity to environmental and use conditions, load rings are generally used as secondary standards to calibrate laboratory or operating standards in well-controlled environments. The precision of this method is dependent on the method of measuring diametrical deflection and its accuracy is dependent on environmental control of temperature.
The third and most common method of calibrating load cells and force gages is the comparison of the components response against some known output, either from a well-characterized testing machine or other force generators. This method is used in various forms in calibration laboratories and production lines. The main constraint in using this method is the maintenance of the working standards, since these systems receive heavy use. This technique relies on close control and observation of the process to maintain accuracy and tends to be susceptible to operator error and to environmental errors, which tend to accumulate with time.